The present invention relates to electrochemical cells which are used as battery power sources by converting chemical energy to electrical energy. More particularly, the present invention relates to a primary or rechargeable electrochemical cell to be used as a regular or rechargeable battery which accomplishes the conversion of chemical energy to electrical energy using a wet (e.g., liquid state) electrolyte, yet maintain a flexible thin layer and open configuration. The present invention further relates to applications of such cell.
The ever-growing development of miniaturized and portable electrically powered devices of compact design such as for example cellular phones, voice recording and playing devices, watches, motion and still cameras, liquid crystal displays, electronic calculators, IC cards, temperature sensors, hearing aids, pressure sensitive buzzers, etc., generates an ever-growing need of compact thin layer batteries for their operation. Therefore, there is a need for reliable thin layer electrochemical cells to be used as batteries in such devices.
Batteries can be broadly classified into two categories in which the batteries of the first category include wet electrolytes (i.e., liquid state batteries), whereas batteries of the second category include solid state electrolyte. Although solid state batteries have an inherent advantage, they do not dry out and do not leak, they suffer major disadvantages when compared with liquid state batteries since, due to limited diffusion rates of ions through a solid, their operation is temperature dependent to a much larger extent, and many operate well only under elevated temperatures; and, the limited diffusion rates thus described, characterize solid state batteries with low ratio of electrical energy generated vs. their potential chemical energy.
Liquid state thin layer batteries typically include a positive and negative active insoluble material layer put together with a separator interposed therebetween, which separator is soaked with a liquid electrolyte solution, thus functioning as an electrolytic liquid layer. Such batteries, an example of which is disclosed for example in U.S. Pat. No. 4,623,598 to Waki et al., and in Japanese Pat. No. JP 61-55866 to Fuminobu et al., have to be sealed within a sheathing film to prevent liquid evaporation, and are therefore closed electrochemical cells. Being closed cells, these batteries tend to swell upon storage due to evolution of gases which is a fatal problem in thin layer batteries having no mechanical support, the pressure imposed by the accumulated gases leads to layers separation, thus turning the battery inoperative. Means to overcome this problem include (i) the use of a polymer increased viscosity agent, such as hydroxyethylcellulose, applied to adhere (i.e., glue) the battery layers together, thus to overcome the inherent problem of such batteries imposed by lack of solid support; and, (ii) addition of mercury to prevent the formation of gases, especially hydrogen. However, the polymer is limited in its effectiveness and the mercury is environmental hazardous.
A way to solve the above described limitation was disclosed in U.S. Pat. No. 3,901,732 to Kis et al. in which a gas-permeable electrolyte-impermeable polymeric material which allows venting of undesirable gases formed within the battery while preventing any electrolyte loss from the battery is used as a sheathing film to enclose the battery cell.
However, a more direct and efficient approach to avoid undesired gas accumulation in liquid state thin layer batteries would be to provide these batteries as open cells for facilitated release of gases, while at the same time to provide means to avoid liquid evaporation and drying out of the battery. Such a construction would permit the production of thin layer batteries devoid of casings, which batteries will therefore be thinner, more flexible and simpler and therefore cheaper for mass production.
There is thus a widely recognized need for, and it would be highly advantageous to have, a flexible thin layer open electrochemical cell devoid of both accumulation of gases and liquid evaporation limitations.